The use of luminous gaseous discharge tubes as a source of light has to date found wide application. Such tubes are generally filled with a pressurized gas and electrically energized to illuminate in a color corresponding to the selected gas within. Each tube, consequently, is effectively capable of displaying only a single color.
More recently, a gas discharge tube capable of lighting in multiple colors has been disclosed in Japanese Patent Publication No. 53-42386. This multi-color gas discharge tube, containing therein two kinds of discharge gases different in excitation energy, is energized with a pulse current. According to the shape and duty cycle of the pulse, either of the gases in the tube is selectively excited to make the tube light in a color characteristic of the excited one of the gases. Additionally, the tube can be made luminous not only in one of the two colors by suitably selecting the constitution of the pulse current in accordance with the excitation energy of the selected gas, but also in any color determined from the combination of the two colors by constituting the pulse current with a suitable combination of two kinds of pulse shapes and duty factors.
The kinds of discharge gases, however, are usually restricted to neon (emitting a light of red) and mercury vapor (emitting a light of blue in the visible light region), so that the luminous light colors are limited to red, blue and a color obtained from the combination of them. Therefore, the prior art gas discharge tube can not be made luminous, for instance, in yellow and green.